This article provides detailed explanations of the Lesson Reports available in PPO:
The report includes all the fields on the Lessons entity for all lesson records and projects. If the “Include Project Information” checkbox is selected, the report will also include all the fields on the project entity.
The Lesson Learnt Report shows a list of all the current lessons that have been captured on the respective projects.
The lessons learnt report is usually used as part of the project closure. During the closure meeting, all the lessons that were learnt are captured on the project.